Frame structure, system and method for OFDM communications

ABSTRACT

A method and apparatus are provided for performing acquisition, synchronization and cell selection within an MIMO-OFDM communication system. A coarse synchronization is performed to determine a searching window. A fine synchronization is then performed by measuring correlations between subsets of signal samples, whose first signal sample lies within the searching window, and known values. The correlations are performed in the frequency domain of the received signal. In a multiple-output OFDM system, each antenna of the OFDM transmitter has a unique known value. The known value is transmitted as pairs of consecutive pilot symbols, each pair of pilot symbols being transmitted at the same subset of sub-carrier frequencies within the OFDM frame.

RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application Nos.60/329,507, 60/329,510 and 60/329,514 all filed Oct. 17, 2001.

FIELD OF THE INVENTION

This invention relates to cellular wireless communication systems, andmore particularly to system access within cellular wirelesscommunication systems employing OFDM or OFDM-like technology, and tophysical layer packet and preamble designs.

BACKGROUND OF THE INVENTION

In a wireless communication system having at least one transmitter andat least one receiver, the receiver must acquire the timing of a signaltransmitted by the transmitter and synchronize to it before informationcan be extracted from the received signal. The timing of signalstransmitted from a base station, within a wireless communication system,is commonly referred to as the system timing.

In cellular wireless communication systems employing OrthogonalFrequency Division Multiplexing (OFDM), synchronization to the timing ofa signal enables the exact positioning of a Fast Fourier transform (FFT)window utilised by a receiver of the signal to extract information fromthe signal.

In any cellular wireless communication system having multiple basestations (BTS) and multiple mobile communication devices thesynchronization process must occur frequently between the BTS and themobile communication devices for the system to be operable. The mobilecommunication devices will simply be referred to hereinafter as UE (userequipment).

Furthermore, each BTS defines a geographic transmission region, knowncommonly as a cell, in which UE in substantially close proximity to aparticular BTS will access the wireless communication system. Theprocess whereby a particular UE selects a BTS from which to access thecellular wireless communication system is known as cell selection. Inorder to optimize the reception of the BTS signal, the UE needs toidentify the best quality signal received from different BTSs and switchits receiver to tune into the best BTS for a given time. Thus, due tothe mobility of UE, the synchronization process has to be employedfrequently in order to allow seamless handoffs from one BTS to anotherBTS as the UE changes location.

In most current cellular wireless communication systems, fast systemaccess and cell selection are essential functions for proper mobile UEoperation. The objective of fast acquisition is to allow UE tosynchronize into the desired BTS. The cell selection and re-selection isperformed by UE to synchronize and measure the signal (including theinterference) power among the adjacent BTS and select and switch to theBTS with the best signal quality, namely the maximum C/I(carrier-to-interference) ratio.

Existing solutions to access a wireless communication system employingOFDM (Orthogonal Frequency Division Multiplexing) were designed forwireless LAN (local area network) systems for fast packet access under aSISO (single input-single output) configuration. However, the wirelessLAN does not have the capability to deal with the UE mobility, whichrequires seamless BTS handoff. On the other hand some cellular systemse.g. 3G UMTS are capable of performing cell selection and BTSidentification and BTS C/I ratio measurement.

Multiple Input Multiple Output-Orthogonal Frequency DivisionMultiplexing (MIMO-OFDM) is a novel highly spectral efficient technologyused to transmit high-speed data through radio channels with fast fadingboth in frequency and in time. For a high-speed downlink packet datatransmission system, the design of the physical layer packet structureis a fundamental aspect.

OFDM technology has been adopted by DAB, DVB-T and IEEE 802.11standards. DAB and DVB-T are used for audio and video territorialbroadcasting. In these systems, the signal is transmitted in acontinuous data stream. A preamble is not needed because fast packetaccess is not critical. DAB and DVB-T are also applied in singlefrequency networks. In this case, every transmitter transmits the samesignal as a simulcast. The interference from the neighbouringtransmitters can be treated as an active echo, which can be handled bythe proper design of the prefix. IEEE 802.11 is the wireless LANstandard. It is a packet based OFDM transmission system. A preambleheader is introduced in this standard.

Synchronization within MIMO-OFDM (Multiple Input Multiple Output-OFDM)systems, in which each transmitter and each receiver have multipleantennae, is even more difficult. Adding to the complexity of the taskis that a fast synchronization process must be very reliable at very lowC/I ratio conditions to allow a high rate of success for the entirecell. In addition, high mobility causes a high Doppler spread and thismakes reliable synchronization even more difficult.

In MIMO-OFDM systems, synchronization can be performed in two steps.First, frame synchronization (also called coarse synchronization) isperformed in order to determine the approximate range of the location ofthe starting position of the first OFDM symbol in the frame. Second,timing synchronization (also called fine synchronization) is performedto determine the precise FFT window location, so that demodulation inthe frequency domain can be performed accurately.

Conventionally, fine synchronization is implemented in time domain. Thisis achieved by inserting an a priori known pilot training sequence inthe time domain for the receiver to perform the cross correlationcomputing at select time slots.

For example, as shown in FIGS. 1A and 1B, the OFDM frame structure ofthe IEEE 802.11 standard utilizes several repeated short OFDM symbolsgenerally indicated at 5 arranged as several headers in the time domainat the beginning of the frame for select sub-carriers, followed bytraining OFDM symbols 207 for fine synchronization. The headers 5 areused for frame (i.e. course) synchronization. The training OFDM symbols207 are used to position the FFT window precisely so that demodulationin the frequency domain can be performed accurately. The training OFDMsymbols 207 are followed by a TPS OFDM symbol 205 and data OFDM symbols30.

The TPS (transmission parameter signalling) OFDM symbol 205, shown moreclearly in the frequency domain (see FIG. 1B), is transmitted with afrequency that corresponds to an adaptive coding and modulation period.The training OFDM symbols, TPS OFDM symbol and data OFDM symbols use allsub-carriers. In the 802.11 system, the repeated headers for coursesynchronization are only transmitted on every fourth sub-carrier. Thisdesign is only suitable for a simple SISO OFDM system with only a singletransmit antenna. For MIMO-OFDM system the preamble design is morecomplicated because of the existence of multiple transmit antennas.Furthermore for mobile communications, an efficient preamble design iseven more difficult because of the multi-cell environment, therequirement for initial access when no BTS information is available, BTSswitching and even soft handoff.

Existing methods in the process of cell acquisition and synchronizationemploy a 3-step-synchronization approach adopted by UMTS WCDMA system,which requires a relatively long access time. While fine synchronizationmay be performed in the time domain, the self-interference of MIMOchannels limits the performance of this approach under very low C/Iconditions. Increasing the length of the correlation can enhance theperformance of fine synchronization in the time domain but at the priceof an increase in overhead and processing complexity. The existingdesigns are based on the time domain training sequence correlation for asingle transmit antenna and a single receive antenna system. However, astraightforward extension of such a time domain synchronization approachwill cause performance loss especially for low C/I ratio applications.The cause of the performance loss is the self-interference between theMIMO channels that is not easy to reduce in time domain.

SUMMARY OF THE INVENTION

One broad aspect of the invention provides a MIMO-OFDM transmitteradapted to transmit a header symbol format in which sub-carriers of aheader OFDM symbol are divided into a non-contiguous set of sub-carriersfor each of a plurality of antennas, with each antenna transmitting theheader OFDM symbol only on the respective set of sub-carriers.

In some embodiments, there are N antennas and a different set ofsub-carriers separated by N sub-carriers is assigned to each of theplurality of antennas.

In some embodiments, the header symbols contain a multiplexed dedicatedpilot channel on dedicated pilot channel sub-carriers and commonsynchronization channel on common synchronization channel sub-carriersfor each of the plurality of antennas.

In some embodiments, the header OFDM symbols further contain multiplexedbroadcasting sub-carriers for each of the plurality of antennas.

In some embodiments, the transmitter is further adapted to transmit apreamble having a prefix, followed by two identical OFDM symbols havingsaid header OFDM symbol format. In some embodiments, the prefix is acyclic extension of the two identical OFDM symbols.

In some embodiments, the pilot channel has a BTS specific mapped complexsequence allowing efficient BTS identification.

In some embodiments, the common synchronization channel is designed forfast and accurate initial acquisition.

In some embodiments, the common synchronization channel is used forcourse synchronization and fine synchronization and the pilot channel isused for fine synchronization.

In some embodiments, the common synchronization channel is used totransmit a complex sequence which is different for each transmit antennaof one transmitter, but which is common for respective transmit antennasof different transmitters within a communications network.

In some embodiments, the transmitter is further adapted to transmit OFDMframes beginning with said preamble, and having scattered pilotsthroughout a remainder of the OFDM frame.

In some embodiments, during the preamble, for each of N transmitantennas, dedicated pilot channel sub-carriers are transmitted andcommon synchronization channel sub-carriers are transmitted andbroadcasting channel sub-carriers are transmitted.

In some embodiments, the sub-carriers of the preamble OFDM symbols areorganized as a repeating sequence of {dedicated pilot channel for eachof N transmit antennas, common synchronization channel sub-carrier foreach of N transmit antennas} arranged in a predetermined order.

In some embodiments, the sub-carriers of the preamble OFDM symbols areorganized as a repeating sequence of {at least one dedicated pilotchannel sub-carrier for each of N transmit antennas, at least one commonsynchronization channel sub-carrier for each of N transmit antennas, atleast one broadcast channel sub-carrier} arranged in a predeterminedorder.

Another broad aspect of the invention provides a MIMO-OFDM receiveradapted to receive a header symbol format in which sub-carriers of aheader OFDM symbol are divided into a non-contiguous set of sub-carriersfor each of a plurality of antennas, with each antenna transmitting theheader OFDM symbol only on the respective set of sub-carriers.

In some embodiments, the receiver is adapted to receive from N transmitantennas with a different set of sub-carriers separated by Nsub-carriers assigned to each of the plurality of transmit antennas.

In some embodiments, the receiver is further adapted to perform finesynchronization on the basis of the common synchronization channelsub-carriers and/or the dedicated pilot channel sub-carriers.

Another broad aspect of the invention provides a transmitter adapted totransmit a packet data frame structure. The packet data frame structurehas a superframe having a length corresponding to a synchronizationperiod of a network; the superframe containing a plurality of radioframes; each radio frame containing a plurality of TPS (transmissionparameter signalling) frames corresponding to an adaptive coding andmodulation period; each TPS frame containing a plurality of slotscorresponding to an air interface slot size; each slot containing aplurality of OFDM symbols, with the first two symbols of the first slotof the first TPS frame of each OFDM frame being used as header OFDMsymbols.

In some embodiments, the header OFDM symbols have a header OFDM symbolformat in which sub-carriers of a header OFDM symbol are divided into anon-contiguous set of sub-carriers for each of a plurality of antennas,with each antenna transmitting the header OFDM symbol only on therespective set of sub-carriers.

In some embodiments, the header OFDM symbols contain multiplexed pilotchannel sub-carriers and common synchronization channel sub-carriers foreach of the plurality of antennas.

In some embodiments, the header OFDM symbols further contain multiplexedbroadcasting channel sub-carriers for each of the plurality of antennas.

In some embodiments, the transmitter is further adapted to transmit in aplurality of different modes by transmitting a different number of OFDMsymbols per slot with an unchanged slot duration and with no change tothe frame structure above the slot.

In some embodiments, wherein modes with an increased number of OFDMsymbols per slot are realized by shortening OFDM symbol duration, andshortening FFT size, but not changing sampling frequency.

In some embodiments, the transmitter is further adapted to transmit to arespective set of users for each TPS frame and to signal for each TPSframe which users should demodulate the entire TPS frame.

Another broad aspect of the invention provides a method of performingsynchronization at an OFDM receiver. The method involves, at each of atleast one receive antenna, sampling a received signal to produce arespective set of time domain samples; determining at least one coursesynchronization position; at each of the at least one receive antenna:

-   -   a) for each of a plurality of candidate fine synchronization        positions about one of said at least one course synchronization        position:        -   i) for each receive antenna positioning an FFT window to the            candidate fine synchronization position and converting by            FFT the time domain samples into a respective set of            frequency domain components;        -   ii) for each said at least one transmit antenna, extracting            a respective received training sequence corresponding to the            transmit antenna from the sets of frequency domain            components;        -   iii) for each transmit antenna, calculating a correlation            between each respective received training sequence and a            respective known transmit training sequence;        -   iv) combining the correlations for the at least one transmit            antennas to produce an overall correlation result for each            candidate synchronization position;    -   b) determining a fine synchronization position from the        plurality of correlation values;

combining the fine synchronization positions from the at least onereceive antenna in an overall fine synchronization position.

In some embodiments, a course synchronization position is determined foreach receive antenna and used for determining the respective finesynchronization position.

In some embodiments, a course synchronization position is determined foreach receive antenna and an earliest of the positions is useddetermining the fine synchronization positions for all receive antennas.

In some embodiments, the course synchronization position is determinedin the time domain for at least one receive antenna by looking for acorrelation peak between the time domain samples over two OFDM symboldurations.

In some embodiments, the method is applied at an OFDM receiver having atleast two antennas, and combining the fine synchronization positionsfrom the at least one receive antenna in an overall fine synchronizationposition comprises selecting an earliest of the fine synchronizationpositions.

In some embodiments, sampling a received signal to produce a set of timedomain samples is done for at least three OFDM symbol durations;determining at least one course synchronization position comprisesperforming a course synchronization in the time domain by looking for acorrelation peak between the time domain samples received over two OFDMsymbol durations to identify a course synchronization position by:

-   -   a) calculating a plurality of correlation values, each        correlation value being a correlation calculated between a first        set of time domain samples received during a first period having        one OFDM symbol duration and a second set of time domain samples        received during a second period immediately following the first        period and having OFDM symbol duration, for each of a plurality        of starting times for said first period;    -   b) identifying the course synchronization position to be a        maximum in said plurality of correlation values.

In some embodiments, combining the correlations for the at least onetransmit antennas to produce an overall correlation result for eachcandidate synchronization position comprises multiplying together thecorrelations for the at least one transmit antenna for each candidatesynchronization position.

In some embodiments, the method is applied to a single transmit antennasingle receive antenna system.

In some embodiments, the training sequence is received on commonsynchronization channel sub-carriers.

In some embodiments, the training sequence is received during an OFDMframe preamble.

In some embodiments, the training sequence is received on dedicatedpilot channel sub-carriers.

In some embodiments, the training sequence is received during an OFDMframe preamble.

Another broad aspect of the invention provides an OFDM receiver havingat least one receive antenna; for each said at least one receiveantenna, receive circuitry adapted to sample a received signal toproduce a respective set of time domain samples; a course synchronizeradapted to determine at least one course synchronization position; afine synchronizer comprising at least one FFT, at least one correlatorand at least one combiner, adapted to, at each of the at least onereceive antenna:

-   -   a) for each of a plurality of candidate fine synchronization        positions about one of said at least one course synchronization        position:        -   i) for each receive antenna position an FFT window to the            candidate fine synchronization position and convert by FFT            the time domain samples into a respective set of frequency            domain components;        -   ii) for each said at least one transmit antenna, extract a            respective received training sequence corresponding to the            transmit antenna from the sets of frequency domain            components;        -   iii) for each transmit antenna, calculate a correlation            between each respective received training sequence and a            respective known transmit training sequence;        -   iv) combine the correlations for the at least one transmit            antennas to produce an overall correlation result for each            candidate synchronization position;    -   b) determine a fine synchronization position from the plurality        of correlation values;

the receiver being further adapted to combine the fine synchronizationpositions from the at least one receive antenna in an overall finesynchronization position.

In some embodiments, the receiver has at least two receive antennas, andis adapted to combine the fine synchronization positions from the atleast one receive antenna in an overall fine synchronization position byselecting an earliest of the fine synchronization positions.

In some embodiments, the receiver is adapted to combine the correlationsfor the at least one transmit antennas to produce an overall correlationresult for each candidate synchronization position by multiplyingtogether the correlations for the at least one transmit antenna for eachcandidate synchronization position.

In some embodiments, the receiver is adapted to receive the trainingsequence on common synchronization channel sub-carriers.

In some embodiments, the receiver is adapted to receive the trainingsequence on dedicated pilot channel sub-carriers.

Another broad aspect of the invention provides a method of performingfine synchronization. The method involves, at each at least one receiveantenna receiving OFDM symbols containing a respective receivedfrequency domain training sequence for each of at least one transmitantenna; performing fine synchronization in the frequency domain bylooking for maximum correlations between known frequency domain trainingsequences and the received frequency domain training sequences.

Another broad aspect of the invention provides a method of transmittingsignals enabling fine synchronization. The method involves from each ofat least one transmit antenna, transmitting OFDM symbols containing arespective frequency domain training sequence.

In some embodiments, a different frequency domain training sequence istransmitted by each transmit antenna, but the same frequency domaintraining sequence is transmitted by corresponding antenna of othertransmitters.

Another broad aspect of the invention provides a method of performingcell selection at an OFDM receiver. The method involves at each of atleast one receive antenna, sampling a received signal to produce arespective set of time domain samples; determining at least one coursesynchronization position; at each of the at least one receive antenna:

-   -   a) performing a frequency domain correlation between at least        one received common synchronization sequence extracted from        common synchronization channel sub-carriers in the received        signal and a corresponding common synchronization sequence of a        respective plurality of transmit antennas to identify a        plurality of candidate correlation peaks;    -   b) selecting the M strongest correlation peaks for further        processing;    -   c) at each correlation peak, reconverting time domain samples        into frequency domain components and processing pilot channel        sub-carriers, these containing transmitter specific information,        to identify a transmitter associated with each correlation peak;    -   d) determining a C/I or similar value for each transmitter thus        identified;

selecting the transmitter having the largest C/I determined for any ofthe at least one receive antenna.

In some embodiments, performing a frequency domain correlation betweenat least one received common synchronization sequence extracted fromcommon synchronization channel sub-carriers in the received signal and acorresponding common synchronization sequence of a respective pluralityof transmit antennas to identify a plurality of candidate correlationpeaks comprises:

-   -   a) for each of a plurality of candidate fine synchronization        positions about one of said at least one course synchronization        position:        -   i) for each receive antenna positioning an FFT window to the            candidate fine synchronization position and converting by            FFT the time domain samples into a respective set of            frequency domain components;        -   ii) for each of at least one common synchronization            sequence, each common synchronization sequence having been            transmitted by a transmit antenna of each of at least one            transmitter, extracting a respective received training            sequence corresponding to the transmit antennas from the            sets of frequency domain components;        -   iii) for each of the at least one common synchronization            sequence, calculating a correlation between each respective            received common synchronization sequence and a respective            known common synchronization sequence;        -   iv) combining the correlations to produce an overall            correlation result for each candidate synchronization            position;    -   b) determining at least one peak in the correlations, each said        at least one peak being local maxima in the correlations.

In some embodiments, the method further involves reconverting timedomain samples into frequency domain components based on the finesynchronization position of the selected transmitter and performing afurther fine synchronization based on a dedicated pilot channel for thattransmitter.

In some embodiments, the method is applied to a MIMO-OFDM frame formathaving a header symbol format in which subcarriers of a header symbolare divided into a non-contiguous set of subcarriers for each of aplurality of antennas, with each antenna transmitting header symbolsonly on the respective set of sub-carriers, and wherein the headersymbols contain multiplexed pilot channel sub-carriers and commonsynchronization channel sub-carriers for each of the plurality ofantennas, the frame beginning with two identical header OFDM symbolsduring which contents of the pilot channel sub-carriers are repeated andcontents of the synchronization channel sub-carriers are repeated, thecommon synchronization channel sub-carriers carrying a complex sequencewhich is different for respective antenna of one base station and beingcommon across multiple base stations, and contents of the dedicatedpilot channel sub-carriers being at least locally unique to a particularbase station.

In some embodiments, the method further involves for transmitterswitching, averaging the C/I or similar value over a time interval foreach transmitter thus identified, and at the end of the time intervalinstigating a transmitter switch to the transmitter with the largestaverage C/I or similar value if different from a currently selectedtransmitted.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will now be described in greaterdetail with reference to the accompanying diagrams, in which:

FIG. 1A is the frame structure of IEEE 802.11 standard in the timedomain;

FIG. 1B is the frame structure of FIG. 1A in the frequency domain;

FIG. 2A is a packet data frame structure provided by an embodiment ofthe invention;

FIG. 2B is a packet frame hierarchy provided by an embodiment of theinvention;

FIG. 3 is a proposed header structure provided by an embodiment of theinvention;

FIG. 4 is a preamble header structure in the time domain provided by anembodiment of the invention;

FIG. 5 is a preamble header structure in the frequency domain providedby an embodiment of the invention;

FIG. 6 is a conceptual schematic view of a MIMO-OFDM transmitterprovided by an embodiment of the invention;

FIG. 7A is a block diagram of a MIMO-OFDM course synchronizationfunctionality;

FIG. 7B is a block diagram of a MIMO-OFDM fine synchronizationfunctionality;

FIG. 8 is a plot of a signature sequence correlation output for pilotchannel showing several candidate synchronization position;

FIG. 9 is a plot of a BTS identification simulation; and

FIG. 10 is a flowchart of a method for cell selection and re-selectionfor MIMO-OFDM provided by an embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 2A, an OFDM packet frame structure provided by anembodiment of the invention is shown. Transmit OFDM symbol streams areorganised into such frames. Each frame consists of three majorcomponents: preamble 300, scattered pilots 302, and traffic data symbols304. The insertion of the preamble allows UE (user equipment) to performthe following fundamental operations: fast BTS (base station) access,BTS identification and C/I ratio measurement, framing and timingsynchronization, frequency and sampling clock offset estimation andinitial channel estimation. The design of a frame preamble withminimized overhead is critical to maximum spectral efficiency and radiocapacity.

Referring now to FIG. 2B, a frame hierarchy for MIMO-OFDM is organizedaccording to an embodiment of the invention as follows: at the highestlevel are OFDM superframes 500 (two shown). The duration of thesuperframe is determined by the network synchronization period (forexample 1-second). The superframe is composed of several 10 ms radioframes 502 also referred to as OFDM frames. There would be 100 10 msOFDM frames 502 in a 1 s superframe 500.

To support adaptive coding modulation (ACM), a fast signalling channel(TPS channel-transmission parameter signalling) is introduced. Each OFDMframe 502 is subdivided into TPS frames 504, in the illustrated examplethere are five 2 ms TPS frames for each 10 ms radio frame 502. The framelength used for TPS in some embodiments is the same as the duration ofthe ACM unit. Each TPS frame also contains signalling information whichallows each user to determine whether the current TPS frame containsdata for them or not. A TPS frame may contain data for multiple users.

The TPS frame 504 can be divided further into several slots 506, each ofwhich consists of several OFDM symbols. In the illustrated example, eachTPS frame 504 is subdivided into 3 slots 506. The duration of the slot506 depends upon the air interface slot size. The smallest transmissionunit is one OFDM symbol 508, 510. The duration of one OFDM symbol isdetermined by the transmission environment characteristics, for example,the maximum channel delay, the system-sampling clock and the maximumDoppler. In the illustrated example, there are four OFDM symbols 508,510 per slot 506.

To reduce the overhead caused by the insertion of the guard intervalbetween OFDM symbols, different OFDM symbol modes each with a differentsymbol duration and a different prefix can be designed, for example, 0.5k mode and 1 k mode. To simplify the system the sampling frequency iskept unchanged when doing the mode switching. These different modes aredescribed in more detail below.

The frame structure of FIG. 2B gives an example of a frame structurehierarchy compatible to the UMTS air-interface. At the OFDM symbollevel, there are two different types of OFDM symbols. These include thepreamble OFDM symbols 508 and regular data symbols 510.

Referring now to FIG. 4, which is a time domain representation, eachOFDM frame starts with a preamble, which consists of several identicalheader OFDM symbols 603, 605 preceded by a prefix 607 which is a cyclicextension of the header OFDM symbols. A repetition structure is used toassist synchronization. By performing a correlation between adjacentOFDM symbols until two identical symbols are identified, the start of anOFDM frame can be found. By way of example, there may be 1056 samplesused per OFDM symbol. For the preamble, during the prefix 607, the last64 samples of the header OFDM symbols are transmitted. There is noprefix for the second header OFDM symbol. The header is insertedperiodically, and for the example of FIG. 2B, this occurs every 10 ms,i.e. at the beginning of every OFDM frame.

Referring again to FIG. 2B, it is noted that for non-header OFDMsymbols, i.e. for the regular OFDM symbols 510, every OFDM symbolpreferably also has a prefix. In “1K” mode, there are 32 prefix samples,and 1024 actual samples representing the FFT size, for a total of 1056samples per symbol. In ½K mode, there is a 16 sample prefix, and then512 samples per symbol (representing the FFT size) for a total of 528samples/symbol. Advantageously, using the frame structure of FIG. 2Bthese different modes can be supported without changing the samplingfrequency. When in ½K mode, there are twice as many OFDM symbols 510 perslot 506. The particular mode chosen at a given instant should be suchthat the prefix size is greater than the maximum channel delay in 1/Kmode, more OFDM symbols are sent with fewer sub-carriers. This is morerobust to high Doppler, because the symbol duration is shorter. Also,the spacing between the sub-carriers is larger further enhancingtolerance to Doppler. Thus, there is a unified frame structure whichaccommodates different FFT sizes, but with the same sampling rate a thereceiver. Preferably the same preamble is used even for the differentmodes.

OFDM is a parallel transmission technology. The whole useful bandwidthis divided into many sub-carriers, and each sub-carrier is modulatedindependently. According to an embodiment of the invention, to separatedifferent antenna with multiple antennas transmission, during the headernot all sub-carriers are used on all transmit antennas. Rather, thesub-carriers are divided between antennas. An example of this will nowbe described with reference to FIG. 3. The sub-carrier frequenciescontained within an OFDM symbol are each represented by circles. In thisexample it is assumed that there are two transmitting antennas in theMIMO system. FIG. 3 shows OFDM symbols with the various sub-carriersspaced along the frequency axis 400, and with the contents of all thesub-carriers at a given instant representing one symbol in time, asindicated along the time axis 402. In this case, the first two OFDMsymbols 408, 410 are used for dedicated pilot channel information whilethe remaining symbols (only two shown, 412, 414) are used for regularOFDM symbols. The dedicated pilot channel information transmitted on thefirst two OFDM symbols 408, 410 alternates by sub-carrier between beingtransmitted by the first antenna and the second antenna. This isindicated for the first sub-carrier 404 which is transmitting dedicatedpilot channel information for the first transmitter and sub-carrier 406which is transmitting dedicated pilot channel information for the secondsub-carrier, and this pattern then repeats for the remainder of thesub-carriers. The other OFDM symbols 412, 414 contain informationtransmitted by both antennas. It is to be understood that other spacingscould alternatively be used. Furthermore, if there are more then twotransmit antennas, the pilot channel information would then alternate bysub-carrier in some predetermined pattern between all of the transmitantennas.

In another embodiment, a common synchronization channel, and dedicatedpilot channel are frequency multiplexed onto the header symbols. Arespective set of non-overlapping sub-carriers are assigned for eachantenna to transmit respective dedicated pilot channel and commonsynchronization channel.

In another embodiment a common synchronization channel, dedicated pilotchannel and a broadcasting channel are frequency multiplexed onto theheader symbols. Under this arrangement, the total useful sub-carriers ofthe header symbols are separated into three groups. These three groupsare mapped onto the common synchronization channel, dedicated pilotchannel and the broadcasting channel respectively.

An example of the mapping of the different channels in the MIMO-OFDMsystem with two-transmitter diversity is shown in FIG. 5. In thisexample, there are shown four OFDM symbols 712, 714, 716, 718 two ofwhich 712, 714 are header symbols. During the header symbols 712,714every second sub-carrier is used for the first antenna with theremaining sub-carriers used for the second antenna. This is easilygeneralized to higher numbers of antennas. For this example, it isassumed that there are two transmit antennas in the MIMO system. Everysixth sub-carrier starting at the first sub-carrier 700 is for the firsttransmitter dedicated pilot channel sub-carriers. Every sixthsub-carrier starting at the second sub-carrier 702 is for the secondtransmitter dedicated pilot channel sub-carrier. Every sixth sub-carrierstarting at the third sub-carrier 704 is for the first transmittercommon synchronization channel sub-carrier. Every sixth sub-carrierstarting at the fourth sub-carrier 706 is for the second transmittercommon synchronization channel sub-carrier. Every sixth sub-carrierstarting at the fifth sub-carrier is for broadcasting channelsub-carriers for the first antenna, and every sixth sub-carrier startingat the sixth sub-carrier 710 is for broadcasting channel sub-carriersfor the second antenna.

The common synchronization channel is a universal channel for initialaccess. It can also be used for synchronization and preliminary channelestimation. The different transmitters share the common synchronizationsub-carriers when transmitter diversity is applied. In which case asindicated above the common synchronization channel is divided betweendifferent transmitters. A common complex sequence known by all theterminals is used to modulate the sub-carriers reserved for the commonsynchronization channel. The same common synchronization sequence istransmitted by all base stations within a system. There may be one ormore such synchronization sequences in the event that there are multipletransmit antennas such that each transmit antenna can transmit a uniquesynchronization sequence. Using the synchronization sequence, mobilestations are able to find initial synchronization positions for furtherBTS identification by looking for a correlation peak between receivedsynchronization sequence and the known transmitted synchronizationsequence.

The dedicated pilot channel is used for BTS/cell identification, andsupports C/I measurement for the cell selection, cell switching andhandoff. A unique complex sequence, for example a PN code, is assignedto each BTS and used to modulate the dedicated pilot sub-carriers. Adifferent unique sequence is transmitted by each antenna in the multipletransmit antenna case. Unlike the case for the common synchronizationchannel, different base stations transmit using different pilotsequences. The quasi-orthogonality of the PN codes assigned to differentBTSs makes it possible to do access point identification and initialinterference measurement. The dedicated pilot channel can also be usedto assist the synchronization processing.

To fully utilize the sub-carriers in the header OFDM symbols, asindicated above, some sub-carriers are preferably used as a broadcastingchannel. In the example of FIG. 5, two of every six sub-carriers areused for this purpose. The broadcasting channel can carry importantsystem information. STTD (space time transmit diversity) schemes cannotbe used for the broadcasting channel (or any of the sub-carriers in theheader OFDM symbols) because of it will destroy the repetition structureof the header OFDM symbols which is required by synchronizationalgorithms. However transmitting the broadcasting information by alltransmitters on the same sub-carrier may cause destructive interferencebetween transmitters. To solve that problem the broadcasting channel ispartitioned between different transmitters, so in the two transmitantenna case, the sub-carriers (mapped for the broadcasting channel) canbe assigned alternatively for the transmit antenna to provide diversity.Power boosting may be applied to further enhance the broadcastingchannel.

The broadcasting information from different BTS's can be different. Insome embodiments broadcasting information is protected so those usersclose to the cell boundaries can receive it correctly in the presence ofstrong interference. A short PN code could be used to spread thebroadcasting information. The neighbouring BTS is assigned to usedifferent code. The insertion of the broadcasting channel reduces thepreamble overhead and increases the spectrum efficiency.

The broadcast channel is used to transmit information unique to theparticular base station. A single broadcast message may be sent on thecombined broadcast channel carriers for the two antennas. By designingthe preamble header symbol to consist of pilot channel, synchronizationchannel and the broadcasting channel, the preamble header overhead isreduced. The common synchronization channel is designed for fast andaccurate initial acquisition. The dedicated pilot channel with a BTSspecific mapped signature allows an efficient BTS identification. Thecombined common synchronization channel and the pilot channel are usedtogether for MIMO channel estimation. The use of the combined commonsynchronization channel and the dedicated pilot channel also allows forhigh accuracy synchronization. Frequency domain training symbols arerobust to timing error and multipath environments. The preamble designallows the flexibility of the user equipment to implement more efficientalgorithms.

It is noted that the specific breakdown of sub-carriers between thededicated pilot channel in one embodiment, between the dedicated pilotchannel and common synchronization channel in another embodiment, andbetween the dedicated pilot channel, common synchronization channel andbroadcast channels in another embodiment, are only specific examples.These can be allocated in any suitable manner.

Referring now to FIG. 6, shown is a conceptual schematic of a MIMO-OFDMtransmitter 10. A first sample set of four OFDM symbols 201 is showntransmitted from a first transmit antenna 21 and a second sample set offour OFDM symbols 203 is shown transmitted from a second transmitantenna 23. In general an OFDM transmitter will have N_(ant) transmitantennae, where N_(ant) is a design parameter. Within the MIMO-OFDMtransmitter 10, data originating from a demultiplexer 23 are sent to oneof either a first OFDM component 24 connected to transmit antenna 21 ora second OFDM component 26 connected to transmit antenna 23. Thecomponents organize the data onto sub-carriers of OFDM symbols and OFDMframes, each sub-carrier being at a different orthogonal frequency. EachOFDM component 24,26 has a respective header inserter 29 which insertsheader OFDM symbols. The sample sets of OFDM symbols 201 and 203represent the first four OFDM symbols of the transmitted OFDM frame fromtransmit antennae 21 and 23, respectively, where each row of datasymbols or pilot symbols is an OFDM symbol. A first OFDM symbol 13 and asecond (identical to the first) OFDM symbol 14 represent the two headerOFDM symbols unique to the OFDM frame transmitted by first transmitantenna 21. Similarly, a third OFDM symbol 17 and a fourth (identical tothe third) OFDM symbol represent the two header OFDM symbols unique tothe OFDM frame transmitted by the second transmit antenna 23. Four OFDMsymbols 15, 16, 19, 20 are typically non-identical OFDM symbols made upof a plurality of data symbols, with at least one data symbol indicatedgenerally at 11 on each OFDM sub-carrier. An entire OFDM frame wouldtypically have many more data symbols. Also, the OFDM symbols 201 aretransmitted concurrently, and with the same timing, as OFDM symbols 203.

In this example, the two identical header OFDM symbols consist ofdedicated pilot channel sub-carriers 12 and common synchronizationchannel sub-carriers 9. There may also be broadcast channelsub-carriers, not shown. The dedicated pilot channel sub-carriers areused for C/I ratio measurement and BTS identification and finesynchronization as detailed below; they can also be used for initialchannel estimation. The common synchronization channel sub-carriers 9are used for course synchronization and fine synchronization, initialaccess, and initial channel estimation.

In the illustrated example, during the two header OFDM symbols, thefirst of every four consecutive sub-carriers is used to carry dedicatedpilot channel symbols transmitted by transmitting antenna 21. Similarly,the second of every four consecutive sub-carriers is used to carrydedicated pilot channel symbols transmitted by transmitting antenna 23.

The dedicated pilot channel symbols transmitted on the pilot channelsub-carriers 12, 25 are defined by base station/sector specific PNsequence. A set of symbols from a complex pseudo-random PN sequenceunique to the base station is mapped onto the dedicated pilot channelsub-carrier locations in the header OFDM symbols.

The third of every four consecutive sub-carriers in the two headersymbols is used to carry common synchronization channel symbolstransmitted by transmitting antenna 21. Similarly the fourth of everyfour consecutive sub-carriers is used to common synchronization channelsymbols transmitted by transmitting antenna 23.

The common synchronization channel symbols transmitted on the commonsynchronization sub-carriers 9, 27 are defined by unique complexpseudo-random PN sequence for each transmit antenna 21 and 23. A set ofsymbols from this complex pseudo-random PN sequence is mapped onto thecommon synchronization channel sub-carriers in the header OFDM symbols.That is, the common synchronization channel symbols of each frametransmitted through each transmitting antenna use a PN code unique tothat transmitting antenna but which is the same for correspondingtransmitting antennas of other base stations. In the present examplePN_(SYNC) ⁽¹⁾ is associated with transmit antenna 21 and PN_(SYNC) ⁽²⁾is associated with transmit antenna 23. However, similar antennae indifferent transmitters throughout the communication network will use thesame PN code. For example, the common synchronization channel symbolsfor a first transmit antenna 21 on all transmitters within the networkwill use one PN code (PN_(SYNC) ⁽¹⁾) and the common synchronizationchannel symbols for a second transmit antenna 22 on all transmitterswithin the network will use a different PN code (PN_(SYNC) ⁽²⁾).

Referring to FIG. 7A, a block diagram of MIMO-OFDM receiverfunctionality is shown which is adapted to perform coarsesynchronization based on the two repeated OFDM header symbolstransmitted by each transmit antenna as detailed above. The OFDMreceiver includes a first receiving antenna 734 and a second receivingantenna 735 (although more generally there will be a plurality of Nreceiving antennae). The first receiving antenna 734 receives a firstreceived signal at RF receiver 736. The first received signal is acombination of the two signals transmitted by the two transmittingantennae 21 and 23 of FIG. 6, although each of the two signals will havebeen altered by a respective channel between the respective transmittingantenna and the first receiving antenna 734. The second receivingantenna 735 receives a second received signal at RF receiver 739. Thesecond received signal is a combination of the two signals transmittedby the two transmitting antennae 21 and 23, although each of the twosignals will have been altered by a respective channel between therespective transmitting antenna and the second receiving antenna 735.The four channels (between each of the two transmitting antennae andeach of the two receiving antennae) may vary with time and withfrequency, and will in general be different from each other.

Coarse synchronization is performed for the first receive antenna 734 bya coarse synchronizer 737 on discrete time samples of a received signalto determine an approximate range of a location of the starting positionof the first header symbol. A similar process is performed by coursesynchronizer 741 for the second antenna 735. Coarse synchronization isfacilitated by the use of repeated header symbols at the OFDMtransmitter. The coarse synchronizer 737 performs correlationmeasurements on time domain signal samples in successive OFDM symbols.The time domain signal sample yielding the highest correlationmeasurement is the coarse synchronization position n_(coarse). Thecourse synchronization position n_(coarse) is then used as the positionon which to locate an FFT window within the FFT functions used in finesynchronization.

Initially, the coarse synchronizer 737 starts the time domain coarsesynchronization processing. A running buffer (not shown) is used tobuffer discrete time samples of the received signal over threesuccessive OFDM symbol period, and then calculates the auto-correlationγ_(t)(n) between samples collected during two successive OFDM symboldurations as follows:${\gamma_{i}(n)} = {\sum\limits_{i = 0}^{{Nheader} - 1}\quad{{x( {n + i} )}{x^{*}( {n + i + N_{header}} )}}}$where x(n) is the time domain samples of the received signal, N_(header)is the number of samples taken over one OFDM symbol duration.

In some embodiments, a moving correlator is applied in the real timeimplementation to save calculation power.

In one embodiment, the values of γ_(t)(n) are calculated in sequence,for n=1 (until n=N_(header)), until a correlation value is above athreshold, after which a maximum search is enabled. The computation ofthe correlation values continues and the maximum search process willcontinue until the correlation result is below the threshold again. Thesample position corresponding to the maximum correlation value is thecoarse synchronization position:n _(coarse)=argmax(|γ_(t)(n)|) nε{γ_(t)(n)>γ_(threshold)}

The threshold is typically calculated from the average auto-correlationvalues within one frame. Alternatively, another way of finding themaximum is to determine a local maximum for each OFDM symbol over anOFDM frame which might be 60 symbols in length for example. Then, theoverall maximum is taken to be the maximum of the local maxima. Thisprocess is conducted both course synchronizers. In the event finesynchronization is to proceed jointly, the overall coursesynchronization position may be taken as some combination of the twosynchronization values, and is preferably taken to be the earlier of twocourse synchronization positions thus determined. Alternatively, eachfine synchronizer (detailed below) can work from a respective coursesynchronization position.

Referring to FIG. 7B, a block diagram is shown of an MIMO-OFDM finesynchronization functionality is shown. In one embodiment, the finesynchronization functionality is adapted to perform fine synchronizationbased on the two-repeated OFDM header symbols transmitted by eachtransmit antenna as detailed above using the common synchronizationchannel and/or the dedicated pilot channel. More generally, the finesynchronization functionality can perform fine synchronization for OFDMframes within which some known training sequence has been embedded.Also, an input to the fine synchronization process is a coursesynchronization position. This course synchronization position may bedetermined using the above discussed method, or using any other suitablemethod. The components which are identical to those of FIG. 7A aresimilarly numbered and in an actual implementation would be shared ifthe common synchronizers of FIG. 7A are to be used. The functionality ofFIG. 7B is replicated for each of the one or more receive antenna.

A fine synchronization process is performed for each of one or morereceive antennae, and then an overall synchronization position is takenbased on a combination of the fine synchronization positions. By way ofoverview, once the coarse synchronizers have determined the coarsesynchronization position(s) n_(coarse), each fine synchronizer performsan FFT on the signal samples on either side of the coarsesynchronization position, to generate frequency domain components overthe frequency band of OFDM sub-carriers. Each fine synchronizer searchesthe frequency domain components in order to locate the precise locationof the FFT window. The precise location of the FFT window is required inorder to perform OFDM demodulation in the frequency domain. The finesynchronizer locates the precise location of the FFT window byperforming correlation measurements between the known PN codes(PN_(SYNC) ⁽¹⁾ & PN_(SYNC) ⁽²⁾) and the frequency components within asearching window defined with respect to the coarse synchronizationposition n_(coarse). The correlation measurements performed by each finesynchronizer are performed in the frequency domain, and one set ofcorrelation measurements is performed for each known PN code (PN_(SYNC)⁽¹⁾ & PN_(SYNC) ⁽²⁾), that is, for each transmitting antenna 21 and 23(or for how many of the one or more transmit antenna there are).

Each fine synchronizer selects N_(symbol) signal samples starting at aninitial signal sample within the searching window, where N_(symbol) isthe number of signal samples in an OFDM symbol. For each transmittingantenna, each fine synchronizer determines a correlation measurementbetween the frequency domain signal samples and the PN codecorresponding to the transmitting antenna.

More specifically, fine synchronization searching is performed nearn_(coarse). Supposing that the searching window is 2N+1, the searchingrange is from (n_(coarse)−N) to (n_(coarse)+N). Letn_(start)(i)=n_(coarse)+N−i represent the sample index within the finesearching window, where i=0, . . . , 2N. The fine synchronization startsfrom i=0. Then N_(symbol) samples are taken starting from n_(start)(0),the prefix is removed and FFT is performed. The received OFDM symbol infrequency domain can be written as:R(l,i)=FFT(x(n(i),l)), n(i)=[n _(start)(i)+N _(prefix) , n _(start)(i)+N_(symbol)−1]; l=1, . . . , N _(FFT):where N_(prefix) is the number of prefix samples and N_(FFT) is the FFTsize.

From R, the complex data R^((j,k)) _(SYNC) carried by the commonsynchronization channel of different transmitters is extracted, sincecommon synchronization channels are divided between differenttransmitters in MIMO OFDM system. More generally, the complex the datacorresponding to a transmitted training sequence is extracted. Thecorrelation between R^((j,k)) _(SYNC) and PN*^((j)) _(SYNC) is:${{{\gamma_{f}}^{({j,k})}(i)} = {\sum\limits_{m = 0}^{N_{SYNC} - 1}\quad{{R_{SYNC}^{({j,k})}( {m,i} )} \cdot {{PN}_{SYNC}^{*{(j)}}(m)}}}},{i = 0},\ldots\quad,{2N}$where j=1, 2, . . . , N_(Tx) indicates transmitter, k=1, 2, . . . ,N_(Rx) indicates receiver, PN^((j)) _(SYNC) is the common SYNC PN codefor j^(th) transmitter and N_(SYNC) is the size of common PN code.

Then the starting point index n_(start) is shifted by one(n_(start)(1)=n_(start)(0)−1), and another N_(symbol) samples areprocessed as described above. In order to get the new frequency domaindata R^((j,k)) _(SYNC) (m,i), we need to perform FFT again. An iterativemethod can be used for this purpose to reduce the computationalcomplexity:R(l,i)=R(l,i−1).e ^(i2π(k−1)/NFFT) +x(n _(start)(i)+N _(prefix))−x(n_(start)(i−1)+N _(symbol)−1)

where NFFT is the FFT size. Extracting R^((j,k)) _(SYNC) (m,i), the newcorrelation is calculated. The above procedure is continued untiln_(start) moves out of the fine searching window.$n_{fine} = {\arg\quad{\max( {\prod\limits_{j = 1}^{N_{T\quad x}}\quad{\prod\limits_{l = 1}^{N_{R\quad x}}\quad{{\gamma_{f}^{({j,k})}(i)}}}} )}}$

For each receive antenna, a respective fine synchronization position canbe found by finding n_(start)(i) 5 corresponding to the maximum of theproducts of the correlation results from different antennas over i=0, .. . , 2N. In mathematical terms, for the kth receive antenna, arespective fine synchronization position can be selected according to:${n_{fine}(k)} = {\arg\quad{\max( {\prod\limits_{j = 1}^{N_{T\quad x}}\quad{{\gamma^{({j,k})}(i)}}} )}}$

To reduce the possibility of false alarm, a criterion may be set. Forexample, the fine synchronization may be considered to be achieved ifthe following condition is satisfied,${\max( {\prod\limits_{j = 1}^{N_{T\quad x}}\quad{{\gamma^{({j,j})}(i)}}} )} > {N_{{threshold} \cdot \frac{1}{{2N} + 1}} \cdot {\sum\limits_{i = 0}^{2N}\quad{\prod\limits_{j = 1}^{N_{Tx}}\quad{{\gamma^{({j,j})}(i)}}}}}$where N_(threshold) is a factor determined by the pre-set fine searchingwindow size. Preferably, an overall fine synchronization position isthen taken to be the earliest of the fine synchronization positionsdetermined for the different receive antennas.

The fine synchronization process for one receive antenna is illustrateddiagrammatically in FIG. 7B. At the output of the first receiver 736,blocks D0 738 through D2N 742 represent alignment of the FFT blocks 744,. . . , 748 for 25 the various candidate fine synchronization positions(2N+1 in all). The FFT blocks 774, . . . , 748 compute an FFT on eachrespective set of samples. Each FFT output is fed to a correlator blockfor each transmit antenna. If there are two transmit antennae, thenthere would be two such correlator blocks per FFT output. For exampleFFT 744 has an output fed to a first correlator block 745 for the firsttransmit antenna, and fed to a second correlator block 755 for a secondtransmit antenna. It is noted that if the spacing of the sub-carriersused to transmit the training sequence (the common synchronizationsequence or pilot channel sequence in the above examples), a full FFTdoes not need to be completed in order to recover the training sequencecomponents. The correlator block 745 for the first antenna multiplieswith multiplier 747 the recovered training sequence symbol locations ofthe FFT output by the known training sequence for the first transmitantenna and these multiplications are added in summer 751. This samecomputation done in correlator 755 for the known training sequence ofthe second transmit antenna and the training sequence locations for thesecond transmit antenna. This is done at the first receiver for all ofthe different possible shifts for each transmit antenna. The correlationresults across different transmit antennas for each possible shift aremultiplied together in multipliers 753. The shift which results in themaximum of these multiplications is selected to be the finesynchronization position for the particular receiver. The same processis followed for any other receive antennas, and the overall finesynchronization position is preferably taken as the earliest of the finesynchronization positions thus computed.

The timing synchronization can be tracked every frame in case that thesynchronization position drifts or losses. For example, in systemsemploying the previously described preamble, each time a preamblearrives at the receiver the 2-step process of synchronization isrepeated, using the same method for coarse synchronization and finesynchronization. In this case, a smaller searching window N may be usedbased on the assumption that the drift of the synchronization positionshould be around the vicinity of the current location. Afteracquisition, the dedicated pilot channel code assigned to modulatededicated pilot channels for different BTS can be used in thecorrelator, or the common synchronization sequence can be used, or someother training sequence.

An embodiment of the invention has been described with respect to anMIMO-OFDM transmitter having more than one transmitting antenna. Themethod of performing synchronization at the OFDM receiver may also beapplied to a signal received from an OFDM transmitter having only onetransmitting antenna, as long as a known training sequence is insertedin the frame by the OFDM transmitter.

Lastly, in the embodiment of the invention described thus far there hasonly been one transmitter having multiple antennae and one receiverhaving multiple antennae. In what follows, the concepts of the inventionwill be broadened to encompass the multi-cellular environment havingmany MIMO-OFDM transmitters and many MIMO-OFDM receivers.

Access in a Multi-Cellular Environment

System access in a multi-cellular environment introduces the new problemof cell selection, as there will be many transmitters transmitting thesame common pilot symbols. In another embodiment of the invention, thepreviously introduced transmit header is used by receivers to performsystems access and cell selection.

During initial acquisition, the UE starts by performing coarsesynchronization. This may be done using the previously describedmethods, or some other method. After one frame duration, the coarsesynchronization position is determined. Fine synchronization searchalgorithm is performed afterwards based on the common synchronizationchannel. Because the data carried by the common synchronization channelare the same for all BTS, several fingers (peaks) can be observed in amulti-cell environment and multi-path fading propagation channels. Thesefingers usually correspond to different BTS and/or different paths.Referring to FIG. 8, shown is an example of fine synchronization (to thecommon synchronization channel) raw output computed in a multi-cellularenvironment as a function of sample index. In the present example thereare five significant fingers 400, 402, 404, 406, and 408. The Mstrongest fingers are chosen and the corresponding positions arelocated, where M is a system design parameter. These positions are usedas candidates for final synchronization and also as the positions uponwhich BTS identifications are made.

The results of FIG. 8 do not allow BTS identification because BTStransmit the same common synchronization sequences. At each candidatesynchronization position, the correlations of the received dedicatedpilot channel sub-carriers and all possible complex sequences (dedicatedpilot PN sequences) assigned to different BTS are calculated to scan forthe presence of all the possible adjacent BTSs. In the multiple transmitantenna case, preferably this correlation is done on the basis of thecombined dedicated pilot PN sequences of the multiple antennas over allof the dedicated pilot sub-carriers to generate a single correlationresult for each index. FIG. 9 shows an example of the relation betweenthe BTS scanning results and the checking points (candidatesynchronization positions). The BTS identification is realized bydetecting the PN code corresponding to the maximum correlation value ateach candidate synchronization position. C/I can be computed based onall correlation results at each checking position. At the initialacquisition stage, the cell selection is determined by selecting the BTSwith the largest C/I ratio. In the present example two BTS areidentified, a first BTS BTS1 and a second BTS BTS2. Withmultiple-antenna receiver diversity, the final decision of the cellselection should be based on the comparison of the highest C/I obtainedby different receiver antennae at a receiver.

To obtain the final synchronization position, fine synchronization isperformed again, but by using the dedicated pilot channel and thededicated complex sequence found through the BTS identification. Asmaller searching window around the fine synchronization position isused. The final synchronization results from different receivers arecompared. The position corresponding to the earliest sample in time isused as the final synchronization position. This step is to reduce thepossibility that a weak path (multi-path) is selected because of theshort-term fading. To reduce the false alarm probability, a threshold isset. This threshold can be the ratio of the finger strength with respectto the final synchronization position and the average of the correlationwithin the search window.

In the normal data processing stage, the fine synchronization and theBTS identification steps are repeated every frame when a new preamble isreceived, but a small set of the candidate PN codes is applied in theBTS scan. After BTS identification, a BTS candidates list can begenerated through searching strong interferences. This list is updatedperiodically, for example every 10 ms, and provides information for BTSswitch and even soft handoff. Certain criteria can be set in order totrigger the BTS switch and soft handoff. To average the impact from thefading, the decision for BTS switching and the soft handoff may be basedon observation during a certain period. The criteria can be thecomparison of the maximum correlation values representing C and thestrongest I. It should be noted that after the cell switch and the softhandoff, the synchronization may also be adjusted by the final step inthe initial access. The overall cell selection and re-selection methodis shown in FIG. 10.

In the first step 600, coarse synchronization is performed for examplebased on the preamble header in the time domain. This involves finding acoarse boundary between each frame by looking for two identical symbols.Correlating samples over adjacent symbol durations until a peak is founddoes this. Step 600 relies on a preamble to a frame beginning with twoadjacent identical symbols.

Next during step 602, at the coarse synchronization peaks, an FFT iscomputed, and a switch to the processing of the common synchronizationchannel in the frequency domain is made. A search window is centered onsync position +/− a certain number of samples. The M strongestcorrelation peaks are selected, as per 604. At this time, it is notknown which BTS each peak is associated with. BTS identification has notyet been determined.

Then at step 606, for each correlation peak, the FFT is again computedand the correlations associated with the fine synchronization procedureare computed using the dedicated pilot channels—these containing a basestation specific complex sequences. This is immediately followed by step608 where the correlation with the BTS identification complex sequencesis made in order to allow an identification of the associated basestations. At step 610, a C/I ratio is computed for each BTS thusidentified. BTS selection and BTS switching is performed on the basis ofthese C/I ratios in step 612. AS indicated above, BTS switching isperformed on the basis of these C/I ratios averaged over some timeinterval.

Finally, for access, the FFT is computed and fine synchronization isapplied to the dedicated pilot channel of the BTS with the largest C/Iratio as per step 614.

BTS initial synchronization performed on the common synchronizationchannel. A BTS specific sequence is embedded in the frequency domain andBTS identification processing is performed in the frequency domainallowing the elimination of MIMO-OFDM inter-channel interference. BTSpower estimation is performed based on the pilot channel for eachMIMO-OFDM BTS. BTS selection is performed based on C/I ratiomeasurements.

The result is improvement of the synchronization and identification ofthe serving BTS in a severe multi-path channel and high interferenceenvironment by joint BTS synchronization and cell selection. Channelestimation may be performed on a combined common synchronization channeland dedicated pilot channel. Criteria are provided for cell switchingand soft handoff by C/I estimation.

In the above example, the access has been performed based on thesynchronization channel and pilot channel embedded in the previouslydiscussed preamble. More generally, the access can be performed withsuch channels embedded in any suitable manner within an OFDM symbolstream.

What has been described is merely illustrative of the application of theprinciples of the invention. Other arrangements and methods can beimplemented by those skilled in the art without departing from thespirit and scope of the present invention.

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 24. Atransmitter adapted to transmit a packet data frame structurecomprising: a superframe having a length corresponding to asynchronization period of a network; the superframe containing aplurality of radio frames; each radio frame containing a plurality ofTPS (transmission parameter signalling) frames corresponding to anadaptive coding and modulation period; each TPS frame containing aplurality of slots corresponding to an air interface slot size; eachslot containing a plurality of OFDM symbols.
 25. A transmitter accordingto claim 57 wherein the header OFDM symbols have a header OFDM symbolformat in which sub-carriers of a header OFDM symbol are divided into anon-contiguous set of sub-carriers for each of a plurality of antennas,with each antenna transmitting the header OFDM symbol only on therespective set of sub-carriers.
 26. A transmitter according to claim 57wherein the header OFDM symbols contain multiplexed pilot channelsub-carriers and common synchronization channel sub-carriers for each ofthe plurality of antennas.
 27. A transmitter according to claim 57wherein the header OFDM symbols further contain multiplexed broadcastingchannel sub-carriers for each of the plurality of antennas.
 28. Atransmitter according to claim 24 adapted to transmit in a plurality ofdifferent modes by transmitting a different number of OFDM symbols perslot with an unchanged slot duration and with no change to the framestructure above the slot.
 29. A transmitter according to claim 28wherein modes with an increased number of OFDM symbols per slot arerealized by shortening OFDM symbol duration, and shortening FFT size,but not changing sampling frequency.
 30. A transmitter according toclaim 24 adapted to transmit to a respective set of users for each TPSframe and to signal for each TPS frame which users should demodulate theentire TPS frame.
 31. A receiver adapted to receive and process OFDMframes transmitted by the transmitter of claim
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 57. A transmitter according to claim 24 wherein the firsttwo symbols of the first slot of the first TPS frame of each OFDM frameare used as header OFDM symbols.
 58. A transmitter adapted to transmitusing: a first mode of operation in which OFDM symbols are constructedfrom N time domain samples; a second mode of operation in which OFDMsymbols are constructed from N/2 time domain samples.
 59. Thetransmitter of claim 58 adapted to perform switches between the firstand second mode subject to a frame size constraint.
 60. The transmitterof claim 58 further adapted to, for each first mode OFDM symbol,transmit an M sample prefix, and for each second mode OFDM symbol,transmit an M/2 sample prefix.
 61. The transmitter of claim 60 whereinN=1024 and M=32.
 62. The transmitter of claim 58 wherein a sampling ratefor the first mode of operation is equal to a sampling rate for thesecond mode of operation such that OFDM symbols in the second mode aretwice as long as OFDM symbols in the first mode of operation.
 63. Atransmitter adapted to transmit using: a first mode of operation inwhich OFDM symbols are constructed using a first FFT size; a second modeof operation in which OFDM symbols are constructed using a second FFTsize.
 64. The transmitter of claim 63 wherein an increased number ofOFDM symbols per slot are realized by shortening OFDM symbol durationfor the second OFDM mode by setting the second FFT size smaller than thefirst FFT size, but not changing sampling frequency.
 65. The transmitterof claim 63 further comprising a prefix inserter that inserts a prefixbefore each symbol, the prefixes for the first mode of operation havinga first number of samples, and the prefixes for the second mode ofoperation having a second number of samples, with a ratio of the firstnumber of samples to the first FFT size being equal to a ratio of thesecond number of samples to the second FFT size.
 66. A transmittercomprising: a first FFT element that has a first FFT size and generatesfirst OFDM symbols with a first duration, each first OFDM symbol havinga respective first cyclic prefix; a second FFT element that has a secondFFT size that is smaller than the first FFT size that generates secondOFDM symbols with a second duration that is shorter than the firstduration, each second OFDM symbol having a respective second cyclicprefix, a duration of each second cyclic prefix being shorter than aduration of each first cycle prefix; the transmitter being adapted totransmit a series of slots with fixed duration, the slots containingfirst OFDM symbols and second OFDM symbols.